1. Field
This disclosure relates to a biochemical sensor using the surface plasmon resonance phenomena that occur at an interface between a thin metal film and a dielectric, and more particularly, to a high-resolution surface plasmon resonance sensor and a system using the same, capable of narrowing the reflectivity curve due to the surface plasmon resonance, increasing the electric field intensity near the metal surface, thereby enhancing the responsivity to an environmental change, and extending a dynamic range of the sensor.
2. Description of the Related Art
Surface plasmons are charge density waves of free electrons which occur on a surface of a thin metal film with which a dielectric forms an interface and propagate along the interface. Amplitude of their enhanced electric field has maximum at the metal surface and decays exponentially away from the interface in a perpendicular direction. The surface plasmon is generally excited through a coupling of evanescent field generated by a p-polarized light. Since a resonance condition in which surface plasmons are excited depends on a change in a surrounding environment adjacent to the thin metal film surface very sensitively, bio- and gas sensors using this have been extensively studied and developed.
Conventional surface plasmon resonance sensor has a basic configuration consisting of a prism and a single thin metal layer placed on the bottom of the prism. The thin metal layer can be directly deposited on the prism basal plane. Otherwise, the thin metal layer is coated on a transparent substrate optically coupled with the prism by using refractive index matching oil. When a p-polarized laser light is incident onto the metal layer from a side of the prism, the laser light is totally reflected back into the prism above a critical angle of incidence. However, at a specific incident angle higher than the critical angle satisfying a phase matching condition where a component of the incident light wave vector parallel to the prism interface equal to a wave vector of the surface plasmon, the incident light is transferred to the surface plasmon mode propagating along the interface between the thin metal layer and a surrounding medium, and this creates a sharp dip in the reflectance curve.
Since the resonance condition in which the surface plasmons are excited depends on the environment very sensitively, operations of a surface plasmon resonance sensor are performed by measuring a change in the reflectance dip curve in response to a change in the refractive index of the surrounding medium. Depending on experimental means and selection of factors for observing the changes, various measuring methods are available. When a monochromatic laser light is used, methods of measuring a change in the resonance angle at which the surface plasmons are excited, or measuring a change in intensity or phase of the reflected beam in a state where an incident angle is fixed to the initial resonance angle, are possible. When a multi-chromatic light source is used, the environmental change can be measured by monitoring a change in the resonance wavelength for a particular incident angle using a spectrometer.
Novel metals such as gold (Au), silver (Ag), and copper (Cu) can be used for the excitation of surface plasmon. These metals have advantage in that the surface plasmon damping is small and the sharp resonance properties are exhibited since their optical properties are defined by a Drude free electron model in the region of visible to near-infrared wavelengths and the optical absorption loss of metal itself is small. In view of optical properties, Ag is the best. However, there are problems in that Ag has poor thermal, chemical and mechanical stabilities, and inferior biocompatibility. On the other hand, Au has excellent environmental durability and biocompatibility, and therefore has been most commonly used. However, the optical properties are worse than those of Ag.
For early diagnosis of diseases and pathogen infection and rapid analysis of air and environmental pollution, development of a high-resolution sensor capable of detecting small molecules with a molecular weight below hundreds of Daltons and a trace level concentration as low as tens of femtomoles (fM) is necessary. It has been known that the conventional surface plasmon resonance sensor which mainly uses a single Au layer with a thickness of about 50 nm has a limitation in resolution.
The resolution of surface plasmon resonance sensor is improved as the linewidth of the reflectance dip curve decreases and a shift in the resonance angle or wavelength in response to the environmental index change increases. With limited available metals, most of current efforts have been concentrated on decreasing the linewidth of the resonance curve. The approaches can be classified into a method of using laser light of near-infrared wavelength, a method of applying an Au/Ag bimetal layer, a method of using coupling between plasmonic modes such as long range surface plasmon, and the like, through an optical design of multi-layered stack for surface plasmon excitation.
However, since these methods still have problems in stability and resolution, a surface plasmon resonance sensor with further enhanced stability and resolution is required.